Although a fixed wrench of the proper size is generally recognized as the most appropriate tool for turning a nut or bolt head, most tool kits include an adjustable wrench of some sort for convenience. The principal disadvantage of adjustable wrenches is that if the jaws are adjusted to snugly fit the nut to be turned, it is difficult to engage the wrench with the nut. Conversely, if the wrench jaws are separated enough to facilitate engagement with the nut, the common result is that the wrench slips off the nut as torque is applied, often damaging the nut and injuring the user of the wrench. If, on the other hand, the wrench is tightened after placement on the nut, it is difficult to remove the wrench without loosening same, such that in working in a confined spaced, which is often the case, the wrench must be repeadedly, and laboriously placed on the nut, tightened, turned through a small angle, loosened, removed, and placed on the nut in a different position.
In order to overcome such difficulties, lever locking wrenches of various types have been devised. Such wrenches are often constructed in a manner similar to lever locking pliers, such as the well known "Vise-Grip" pliers, wherein an operating lever is releasably latched by an over-center locking mechanism as the lever is closed toward the handle thereby clamping the jaws onto a workpiece. In lever locking wrenches, the jaw faces are generally planar and means is provided for maintaining a parallel relationship of the jaws, usually by mounting the movable jaw in a linear guide track. The advantage of adjustable lever locking wrenches over other types of adjustable wrenches is that when the operating lever is released, there is sufficient clearance between the jaws to facilitate placement about the nut. When the operating lever is closed, the jaws are positively clamped against the nut thereby preventing slippage of the wrench from the nut.
In most of the known types of lever locking wrenches, the spacing of the jaws is adjusted by means of a bolt having a knurled head at the end of the handle, opposite the jaw end of the wrench. Such placement of the adjustment bolt, while feasible as an adjustment means, requires that both hands of the user be employed to adjust the spacing of the jaws. A more convenient adjustment means is a worm and rack gear arrangement placed at the jaw end of the wrench. In one known worm adjustable lever locking wrench, the movable jaw is slidably mounted in an arm which is pivotally connected to the handle having the fixed jaw thereon. A worm mounted on the arm engages a rack on the movable jaw. The wrench also includes an adjustment bolt at the end of the handle. While the two adjustment means provide for a wide range of spacings of the jaws, it is doubtful if the jaw faces (shown as toothed) would remain parallel at the extremes of the adjustment range of the bolt. Further, the worm adjustment structure at the jaw end is rather bulky which would limit the utility of the tool in confined spaces. In another known worm adjusted lever locking wrench, an over-center latched set screw bears against the spindle of the worm gear to slide the movable jaw into clamping engagement with a workpiece. The tool appears quite useful in most respects. However, the latch release lever appears to be inconveniently close to the wrench handle such that the two hands of the user would probably be required for operation of the release lever.